1. Field of the Invention
The present invention relates to capacitive sensors, and more specifically relates to a structure suitable for detecting displacements around two axes on the basis of capacitances.
2. Description of the Related Art
With respect to capacitive sensors which use a capacitance change, tilt sensors for detecting a tilt are known in the art.
A typical tilt sensor includes a pair of electrodes which face each other, one of the electrodes being formed on a fixed plate and the other one being formed on a flexible plate. A weight is attached to the flexible plate, and when the sensor tilts, the flexible plate is deformed due to the weight so that the capacitance between the above-described electrodes changes. The tilt is detected by determining the capacitance change.
However, in the above-described construction in which the elastic distortion of the flexible plate is used, the flexible plate can be moved only slightly and the capacitance cannot be changed by a large amount. Accordingly, sufficient sensor sensitivity cannot be obtained, and the application of the sensor is limited.
In view of the above-described situation, an object of the present invention is to provide a capacitive sensor in which a capacitance changes by a large amount.
In order to attain this object, a capacitive sensor according to the present invention includes a retaining member; an intermediate member retained by the retaining member such that the intermediate member can rotate around a first axis; a movable member retained by the intermediate member such that the movable member can rotate around a second axis which is perpendicular to the first axis; a weight provided on the movable member; a movable electrode unit consisting of one or more electrodes which is provided on the movable member; and a board on which a fixed electrode unit consisting of one or more electrodes is provided such that the fixed electrode unit faces the movable electrode unit. At least one of the movable electrode unit and the fixed electrode unit includes a plurality of electrodes, and the rotation of the movable member is detected on the basis of capacitances between the one or more electrodes of the movable electrode unit and the one or more electrodes of the fixed electrode unit change in accordance with the movement of the movable member.
Accordingly, the movable member can rotate around the fist axis and the second axis independently due to the retaining member and the intermediate member. Thus, the movable member can easily rotate and the displacement of the movable electrode unit with respect to the fixed electrode unit due to the rotation can be increased, so that the tilt, etc., around two axes can be detected with high sensitivity.
Preferably, the retaining member and the intermediate member are connected to each other with a pair of first connecting members which extend along the first axis, the intermediate member and the movable member are connected to each other with a pair of second connecting members which extend along the second axis, and the retaining member, the intermediate member, the movable member, the first connecting members, and the second connecting members are integrally formed of a flexible plate.
According to this construction, the retaining member, the intermediate member, the movable member, the first connecting members, and the second connecting members can be manufactured by simply forming slits in a single plate. Thus, the above-described members can be easily manufactured and manufacturing accuracy can be increased.
The retaining member and the intermediate member may be separated from each other with first slits which are arranged at positions excluding the positions where the first connecting members are disposed, and notch portions may be formed in the retaining member at both ends of the first slits. In such a case, the lengths of the first connecting members are increased due to the notch portions formed in the retaining member at the ends of the first slits, so that the intermediate member can more easily rotate around the first axis. In addition, permanent deformation does not easily occur even when an undesirable external force is applied and the first connecting members are twisted by a large amount. In particular, in the case in which the notch portions are formed only in the restraining member so that the width of the intermediate member is not partially reduced, torsional deformation can be uniformly distributed over the entire area of the intermediate member, and plastic deformation of the intermediate member due to stress concentration can be prevented.
In addition, the intermediate member and the movable member may be separated from each other with second slits which are arranged at positions excluding the positions where the second connecting members are disposed, and notch portions may be formed in the movable member at both ends of the second slits. In such a case, the lengths of the second connecting members are increased due to the notch portions formed in the movable member at the ends of the second slits, so that the movable member can rotate more easily. In addition, permanent deformation does not easily occur even when an undesirable external force is applied and the second connecting members are twisted by a large amount. In particular, in the case in which the notch portions are formed only in the movable member so that the width of the intermediate member is not partially reduced, torsional deformation can be uniformly distributed over the entire area of the intermediate member, and plastic deformation of the intermediate member due to stress concentration can be prevented.
The movable member may be composed of a conductive material, and the movable electrode unit may be integrated with the movable member. In such a case, it is not necessary to form the movable electrode unit separately and the number of manufacturing processes can be reduced, so that the costs can be reduced.
In addition, the movable member may have a plurality of holes at positions close to a bonding area between the weight and the movable member in the peripheral region of the bonding area. In such a case, when the weight is bonded to the movable member by, for example, crimping or welding, distortion of the plate which occurs in the bonding process can be reduced by the holes formed at positions close to the bonding area in the peripheral region of the bonding area.
The capacitive sensor according to the present invention may further include a gap-maintaining means which maintains a gap between the fixed electrode unit and the movable electrode unit. In such a case, the gap between the electrode units can be maintained constant and the detection accuracy can be improved.
The gap-maintaining unit may include a retaining projection which retains the movable member above the board such that the movable member can rotate. In such a case, the movable member is prevented from being deflected toward the substrate. Thus, the tilt of the sensor, etc., can be calculated without taking the vertical displacement of the movable member due to the weight of the weight into account, so that the calculation can be simplified. Alternatively, the gap-maintaining unit may include a plate which surrounds the fixed electrode unit.
In addition, the gap-maintaining unit may include a conductive member through which an electric signal is supplied to the movable electrode unit. In such a case, it is not necessary to provide an additional electric line for supplying the electric signal. Accordingly, the parasitic capacitance of such an electric line is not generated, so that the detection accuracy can be improved.
The capacitive sensor may further include a conductive cover which is grounded and which covers at least the weight, the movable member, and the fixed electrode unit. In such a case, the influence of external noise, etc., can be eliminated.
The shape of the cover is preferably symmetric about a line which extends perpendicularly to the movable electrode unit and passes through the center of the movable electrode unit. In such a case, an initial offset capacitance is not generated and the detection accuracy can be improved.
The capacitive sensor may further include a packing which is disposed between the cover and the board. In such a case, foreign matter, flux, moisture, etc., can be prevented from flowing to the inner region of the cover.